Diesel engines have no inherent braking effect like that experienced with spark ignition engines. The reason is that diesel engines do not have a throttle, which, when closed, causes an increase in intake manifold vacuum to retard the rpm of the engine.
It was first proposed in C. L. Cummins U.S. Pat. No. 3,220,392 to operate a diesel engine in such a manner that the engine produces a retarding effect when the engine is in a motoring condition (fuel to the engine is cut off).
The principle on which the compression relief engine braking system relies is that the energy required by the engine to compress air during the compression stroke is discharged and wasted by opening an exhaust valve at the end of the compression stroke. Since the engine is motoring, the compression stroke is no longer followed by a power stroke so that no energy is generated at any time in the engine cycle. The engine therefore acts as an air pump which discharges the air that it compresses into the exhaust system and thereby uses up the kinetic energy of the vehicle in heating intake air.
The Cummins patent describes a hydraulic mechanism which utilizes the cam motion of a unit injector fuel system to selectively actuate the exhaust valve at top dead center TDC. For engines not utilizing a unit injector fuel system, a lost motion camshaft may be proposed, like the one in Pellizoni U.S. Pat. No. 3,786,792. When this type of lost motion mechanism is applied to an engine with multiple exhaust valves and a floating crosshead, the increased clearances may permit the crosshead to float and become disconnected from the valves.